I think we will need more details.
What exactly does the field look like before you save it?
Is it really Date or Text?
What is the format of the cell?
How are you saving it?
How are you opening it (are you opening in Excel or a Text Editor)?
Hint: You should NEVER use Excel to open CSV file if you really want to see what is in there. This is because Excel does some automated conversions on the data. If you REALLY want to see what the data looks like in the CSV, open with a Text Editor like NotePad, WordPad, or some other text editor program.
If you need to open it Excel to work on it, open it this way:
Go into Excel first
Then go to the Data menu
Select Get External Data, and select the CSV/Text option (may vary slightly in different versions of Excel)
This should invoke the Import Text Wizard, where you can set the data type for each field.
If you do not want this field to change at all, use Text (instead of General or Date).